1. Field of the Invention
The present invention relates to a terahertz wave detection device and method for detecting a weak terahertz wave by using a nonlinear optical effect.
2. Background Art
The term “terahertz wave” means an electromagnetic wave whose frequency is in the range of 0.1 to 10 THz (1 THz=1012 Hz), in other words, whose wavelength is the range from 0.03 to 3 mm of millimeter-wave to a wavelength of far-infrared.
The terahertz wave is expected to be applied in a wide range of fields extending from basic research such as radio astronomy, materials science, and biomolecular spectroscopy to practical applications such as security, information communication, environment, and medical care.
The terahertz wave, however, is an electromagnetic wave having a frequency band between light such as mid and near infrared radiation, visible light, and UV radiation (frequency: 1×1013 to 1015 Hz) and a radio wave (frequency: 103 to 1012 Hz), which leads to a problem that it is impossible to directly apply the existing techniques of optics and electronics to the terahertz wave.
A detection means for detecting a terahertz wave by using the nonlinear optical effect is disclosed in, for example, Patent Literatures 1 to 3.
Moreover, the documents related to the present invention are disclosed in, for example, Non-patent Literatures 1 to 5.
Non-patent Literature 1 is a document related to the generation of a terahertz wave using periodically poled device and differs from the present invention.
Moreover, Non-patent Literatures 2 to 5 relate to the detection of a terahertz wave using a bulk crystal and differ from the detection of a terahertz wave using the periodically poled device.
“A monochrome wavelength variable type terahertz wave generation/detection system and a method” in Patent Literature 1 are terahertz wave detection means using a bulk crystal having a nonlinear optical effect.
“A terahertz wave generation element, a terahertz wave detection element, and a terahertz time domain spectral instrument” in Patent Literature 2 are intended to detect a terahertz wave by irradiating a bulk crystal with an ultrashort pulse laser beam having a shorter pulse width than a picosecond (10−12 second).
“A terahertz photodetector and optical equipment” in Patent Literature 3 use a crystal (photonic crystal) in which layers having different refractive indices are alternately combined.
Moreover, as terahertz wave detectors other than those using the nonlinear optical effect, there are known a heat detection type terahertz wave detector, a terahertz square detector, and a quantum detector.
The heat detection type terahertz wave detector corresponds to a bolometer, a pyroelectric effect detector, or a Golay cell, which detects a terahertz wave as heat energy. This type of detector is quite different from the present invention which uses the nonlinear optical effect. Typical thermal type detectors are as described below.
A detector such as a silicon bolometer operated under very low temperature of 4K has relatively high detection sensitivity. Such detector, however, needs to use liquid helium and therefore cannot be used in a versatile manner in practical application. On the other hand, a pyroelectric effect detector and a Golay cell operate at room temperature, but are lower in the detection sensitivity by more than two digits than a bolometer and have difficulty in enabling a high output of a terahertz wave light source, which often leads to a problem in use.
Moreover, these detectors basically have a low response speed such as microseconds to milliseconds and therefore cannot be used for advanced measurement such as time-resolved spectroscopy.
The terahertz square detector corresponds to, for example, a Schottky diode.
A Schottky diode with a semiconductor such as GaAs capable of operating at high speed is a terahertz wave detector which operates at room temperature and is capable of measuring a pulse whose duration is shorter than a nanosecond. Having a structure of detecting a terahertz wave via an antenna, however, the Schottky diode largely depends on the performance of the antenna. Particularly, the antenna is designed so as to be optimal in a specific frequency region, and therefore a terahertz wave cannot be detected with high efficiency over a wide band such as 1 to 3 THz. Moreover, the wavelength of the terahertz wave is short compared to a microwave or the like, and is several hundreds μm or less and therefore the terahertz wave is reduced in amplitude according to the wavelength, which causes an error at the time of manufacturing to significantly affect the performance.
Furthermore, the Schottky diode responding to high frequency has a whisker antenna and is used with the needle-like antenna in contact with a detector. In this mechanism, the contact cannot be maintained in some cases by a mechanical shock caused by vibration or the like, which leads to a problem for stable terahertz wave measurement.
The quantum detector corresponds to a quantum dot detector, a semiconductor photoconductive detector, or the like.
The quantum detector has high sensitivity and high response speed. The quantum detector, however, operates under very low temperature and therefore is not used in a versatile manner in practical application. The use of the quantum detector is limited to a narrow application such as application to astronomy requiring an ultimate performance.